The present invention relates generally to the grounding system for electrical wiring devices and, more particularly, to electrical switch devices having non-metallic mounting straps and an automatic grounding system for grounding metallic wall plates intended for use with the device to a grounded metallic electrical outlet box, while including a ground terminal for direct termination to the ground conductor of an electrical power cable.
Electrical switches for residential, hospital and industrial use are well known in the art. Hospital and industrial grades of switches are often made in specialized configurations or with heavy duty features in order to satisfy various code requirements (e.g., Underwriters' Laboratories Hospital Grade Test Program). Residential grade switches, on the other hand, are generally required to satisfy somewhat less stringent U.L. requirements and have been manufactured for years with little or no design changes so as to sell at the lowest possible cost. For example, such switches are often made with a body and cover member formed from a thermo-set material using the same type of compression molds which have been in use for decades.
Conventional electrical switches use steel mounting straps (or "ears") to mount the switch to an electrical outlet box, and as a means for grounding the metallic wall plate typically used to cover the switch installation. The steel mounting straps are typically either sandwiched between the switch cover and body, wrapped around the switch device, or fabricated into a continuous metal strip adapted to fit over the switch cover. In such installations, the ends of each strap are provided with mounting screw receiving apertures through which the device can be mounted to the outlet box. The portions of the mounting strap defining each receiving aperture electrically engage the appropriate mounting screw to establish a ground path between a pre-grounded outlet box (metallic) and the mounting strap. Additionally, the straps are typically tapped with small holes to accommodate wall plate mounting screws. The portions of the mounting straps defining each hole electrically engage the wall plate screw to complete the ground path and ground path the wall plate.
Although known prior switch devices have provided generally satisfactory results over the years, they do suffer several disadvantages. For example, the typically used thermoset materials are relatively brittle and are susceptible to cracking or breakage during fabrication or installation. The assembly method of these devices is also a source of problems. The body and cover, with the mounting strap held between them, are usually secured together by a fastening rivet or screw-like fastener which is inserted through the back of the switch body, through a clearance hole formed in the mounting strap and then force fit in a hole formed in the switch cover. To insure adequate holding power, close tolerances are required between the outer diameter of the rivet or screw-like fastener and the holes in the switch body and cover. Thus, if blisters are formed in the vicinity of these holes during molding or if the parts are warped in that vicinity, the resultant parts are susceptible to cracking when the fastener is installed. Further, since the only means of securing the parts together is the rivet or screw fastener, the attachment of the cover and body is vulnerable to loosening, resulting in a dangerous intermittent condition in, or total disconnection of, the electrical connections within the device.
Another disadvantage of conventional electrical switches is the problem encountered when the wall opening cut to receive the receptacle is too large for one or both ends of the mounting strap to bear against the wall surface when the device is mounted to an electrical outlet box behind the wall. To compensate, electricians often rely on the device mounting screws to hold the switch in place. Such installation results in what is called a "floating" installation, wherein the device mounting screws are not tightened all the way so the device can "float" relative to the outlet box. Such "floating" installations are undesirable because they require extra installation time. The electrician must judge how far to tighten down the mounting screws to accommodate the wall plate, yet allow enough slack to pull the device back against the wall plate.
In a proper installation, the ends of both mounting straps should be sufficiently elongated to abut the wall and thus prevent the switch from floating. In the fabrication of steel mounting straps, however, this becomes prohibitively expensive. While the use of cheaper, non-metallic materials to construct the mounting straps may provide an economical alternative, it presents the concomitant problems of (1) providing a mounting strap of durable construction, and (2) providing a means for grounding the wall plate without relying upon the electrically conductive steel mounting strap.
It is therefore an object of the present invention to provide a new and improved electrical switch device which overcomes the foregoing drawbacks and is of rugged construction and capable of economical fabrication, particularly for residential grades. It is another object of the invention to provide a new and improved electrical switch device which can be fabricated essentially completely from thermo-plastic (including mounting strap portions) except for the electrical contacts/terminals and ground elements.
It is also an object of the invention to provide a new and improved electrical switch device having mounting means adapted to substantially reduce the possibility of "floating" installations. In addition, it is an object of the invention to enable incorporation of such mounting feature in switches for various sized wall plates.
It is also an object of the present invention to provide a new and improved electrical switch device having a grounding system capable of automatically grounding a metallic wall plate intended for use with the device when it is mounted to a grounded metal electrical outlet box without requiring a metallic mounting strap. It is a further object of the invention to provide such a switch device which also incorporates alternate grounding means for direct termination to the ground conductor of an electrical power cable for installations where non-metallic outlet boxes are used.
It is yet another object of the present invention to provide an electrical switch device having a cover member and body which can be formed by simple two-part injection molding techniques. It is still another object of the invention to provide such a switch device having a mounting strap formed integrally with the cover member to simplify fabrication and reduce the number of component parts for the complete device.
Further, it is an object of the present invention to provide a new and improved electrical switch device capable of fully automated assembly by the use of component parts which can be secured in place before the final assembly step. It is another object of the invention to provide such a switch device and method of assembly which obviates the need for using the usual rivet or screwtype fastener yet forms a securely assembled switch.
Objects and advantages of the invention are set forth in part herein and in part will be apparent herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the structures, instrumentalities and combinations pointed out in the appended claims. Accordingly, the invention resides in the novel parts, structures, arrangements, combinations, and improvements herein shown and described.